ABSTRACT As the world's population ages, there is an increasingly urgent need to understand the aging process and its role in age-onset disorders. While aging is the greatest risk factor for the development of neurodegenerative disease, the role of aging in these diseases is not well defined. Our recent work suggests that targeting pathways involved in aging may be an effective strategy in the treatment of Parkinson's disease, as well as other neurodegenerative diseases. As the biology of aging is still poorly understood, the goal of the current proposal is to gain insight into the aging process by defining the molecular mechanisms involved in one specific pathway of lifespan extension. We have shown that in C. elegans mild elevation of reactive oxygen species (ROS) levels through deletion of the mitochondrial superoxide dismutase gene sod-2, or directly through treatment with the superoxide-generating compound paraquat, extends lifespan. Importantly, increasing ROS has also been shown to extend lifespan in yeast and mice, indicating conservation across species. However, the mechanism by which ROS increase lifespan is currently unknown. The long-term goal of this work is to advance our understanding of the aging process and to use that knowledge to promote healthy aging and to develop treatments for age-onset neurodegenerative diseases. In the current proposal, the objective is to define the molecular mechanisms by which ROS act to increase lifespan. Our preliminary data demonstrates that the effect of ROS on lifespan is dependent on the precise conditions under which ROS are altered. Increasing mitochondrial superoxide extends longevity, while cytoplasmic superoxide decreases lifespan. Similarly, within the mitochondria, mild increases in superoxide levels increases lifespan, while higher levels of superoxide are toxic. Accordingly, the current proposal will define the conditions required for ROS to increase lifespan, and use this information to elucidate the underlying mechanisms through the completion of three Specific Aims: 1) Determine the mechanism by which elevated mitochondrial superoxide is detected in the cytoplasm; 2). Define the tissues in which mild elevation of mitochondrial superoxide is necessary or sufficient to increase lifespan; and 3) Identify cellular changes induced by elevated mitochondrial superoxide, but not cytoplasmic superoxide, that contribute to longevity. The expected outcome of this project is the elucidation of the mechanism by which mild elevation of mitochondrial superoxide levels extends lifespan. This research is innovative in addressing deficiencies in conventional aging theories by investigating the poorly explored beneficial effect of ROS on lifespan. The work is significant in advancing our molecular understanding of the aging process. This work has the potential to impact public health through the identification of targets for the promotion of healthy aging and treatment of neurodegenerative disease.